As a method for manufacturing carbon-fiber bundles, a conventionally known method is carried out by converting a carbon-fiber-precursor fiber bundle (hereinafter, may also be referred to as a “precursor-fiber bundle”) made of acrylic fibers or the like into a stabilized fiber bundle by heating the bundle at 200 to 400° C. in an oxidizing atmosphere (stabilization process), and by carbonizing the bundle at 1000° C. or higher in an inert atmosphere (carbonization process). Carbon-fiber bundles obtained by such a method exhibit excellent mechanical properties and are widely used as reinforcing fibers especially for composite materials.
However, in manufacturing methods of carbon-fiber bundles during stabilization and subsequent carbonization processes (hereinafter, a stabilization process and a carbonization process may be combined and referred to as a “calcination process”), problems may occur such as fuzzy fibers or yarn breakage because single fibers are fused during a process for converting a precursor-fiber bundle to a stabilized fiber bundle. As a method for preventing single fibers from fusing, applying an oil agent composition on surfaces of precursor-fiber is known (oil treatment), and various oil agent compositions have been studied.
Silicone-based oil agents that contain a silicone as the main component have been used as oil agents in oil agent compositions so as to prevent fusion among single fibers. As the silicones, modified silicones with reactive groups, such as aminos, epoxies and polyethers, have been generally used because of their affinity with and retention on precursor-fiber.
However, when silicone-based oil agents are heated, crosslinking reactions progress to cause high viscosity, and such viscous agents tend to be deposited on surfaces of fiber transport rollers and guides used for manufacturing process and stabilization process of precursor-fiber bundles. Accordingly, the precursor-fiber bundles or stabilized fiber bundles may be wound around or snagged on fiber transport rollers or guides and cause yarn breakage, thus lowering operational efficiency accordingly.
Moreover, during the calcination process, a precursor-fiber bundle with applied silicone-based oil agent tends to produce inorganic silicon compounds such as silicon oxide, silicon carbide and silicon nitride, and to lower industrial productivity.
In recent years, while even larger-scale production facilities and higher productivity have been required in response to an increase in demand for carbon fibers, one of the issues to be solved is a decrease in industrial productivity caused by formation of inorganic silicon compounds during the calcination process.
Accordingly, oil agent compositions with a reduced silicone content have been proposed in an attempt to reduce the amount of silicone in oil-treated precursor-fiber bundles; for instance, in an oil agent composition, the silicone content is lowered by adding 40 to 100 mass % of an emulsifier that contains a polycyclic aromatic compound at 50 to 100 mass % (see Patent Literature 1.)
Another oil composition has been proposed, in which a silicone is blended with a heat-resistant resin having a residual rate of 80 mass % or higher after being heated in air atmosphere at 250° C. for 2 hours (see Patent Literature 2).
In addition, an oil composition has been proposed, in which the silicone content is reduced by adding 80 to 95 mass % of esterified ethylene oxide and/or propylene oxide adducts of bisphenol A esterified with higher fatty acid at both ends (see Patent Literature 3).
Other examples are an oil agent composition made by combining a bisphenol A based aromatic compound and an amino-modified silicone (see Patent Literatures 4 and 5), and an oil agent composition mainly composed of a fatty acid ester of an alkylene oxide adduct of bisphenol A (see Patent Literature 6).
Meanwhile, it has been also proposed to form an oil composition with a lower silicone content by using a compatibilizer so that affinity is enhanced when silicone-based and non-silicone-based compounds are mixed (see Patent Literature 7). Another oil composition has also been proposed, which contains as essential components an ester compound having at least three ester groups in the molecule and a silicone-based compound (see Patent Literature 8). In such an oil composition, the silicone content is reduced by using an ester compound while preventing fusion among single fibers and achieving stable operational efficiency in the production of carbon fibers.
Moreover, by combining an ester compound containing at least three ester groups in the molecule and a water-soluble amide, the silicone content is lowered while fusion of fibers is prevented and stable operational efficiency is achieved (see Patent Literature 9).
Further proposed is an oil agent composition which contains at least 10 mass % of a compound having a reactive functional group but does not contain a silicone compound, or even if the oil agent composition contains a silicone compound, its content is 2 mass % or lower in terms of silicon mass (see Patent Literature 10).
Yet further proposed is an oil agent composition which contains 0.2 to 20 wt. % of an acrylic polymer having an aminoalkylene group in the side chain, 60 to 90 wt. % of a specific ester compound and 10 to 40 wt. % of a surfactant (see Patent Literature 11).
Yet further proposed is to use multiple oil agents when forming an oil agent for carbon-fiber-precursor acrylic fibers (see Patent Literature 12).
Moreover, an oil agent and oil agent composition have been proposed which contains at least one compound selected from a group consisting of specific ester compounds such as hydroxybenzoate and cyclohexanedicarboxylate (see Patent Literatures 13 and 14).